1. Field of the Invention
The present invention relates to a tabular diamond crystal having superior properties useful as an electronic material, and a method for its formation.
2. Related Background Art
Diamonds have a great band gap (5.5 ev), a great carrier mobility (electrons, 1,800 cm.sup.2 /V.S; positive holes, 1,600 cm.sup.2 /V.S) and a great thermal conductivity (20 W/cm.multidot.K) and also have a high hardness and superior wear resistance which are properties that can not be achieved by other materials.
In recent years, researches are being made on syntheses of diamonds from gaseous phases, in particular, on syntheses thereof by chemical vapor deposition (CVD).
Conventional diamonds formed on substrates by CVD include the following:
(1) Those formed by homoepitaxial growth on natural or artificial diamond crystals serving as substrates, and those formed by heteroepitaxial growth on cubic crystal boron nitride (c-BN) having a crystal structure similar to that of a diamond crystal. Both of these have epitaxial relationship to the underlying substrates, and monocrystalline films with a very high flatness can be obtained. PA1 (2) In the formation of diamonds under usual conditions on substrates including a silicon substrate, a high-melting metal such as molybdenum, tungsten or tantalum and a quartz substrate, diamonds are deposited in different forms depending on nucleation density, film thickness and other synthesis conditions. PA1 (3) There are those formed by heteroepitaxial growth on nickel or cobalt substrates. On these substrates which are capable of forming solid solutions with carbon, diamonds partially grow epitaxially. Here, diamond crystals are deposited in a granular form at a small nucleation density, so that no uniform films are formed. PA1 (1) The monocrystalline films obtained by epitaxial growth on diamond or cubic crystal boron nitride substrates can be flat and have a good crystallinity,.but the substrates are so expensive that they can not be of practical use. PA1 (2) In the formation of diamonds under usual conditions on silicon, high-melting metal or quartz substrates; PA1 (3) In the case of the heteroepitaxial growth on nickel or cobalt substrates, although monocrystal grains having epitaxial relationship to the substrates can be obtained, they are deposited in a granular form (the ratio of height to breadth is 1:3 or less, and typically 1:2 or less). In particular, the {111} planes tend to become rough, and a number of crystal imperfections are present in the crystal.
i) In the case when the nucleation density is low, diamond crystals are deposited in a granular form and at random in regard to orientation. PA2 ii) In the case when the nucleation density is high, diamond crystals are deposited in the form of polycrystalline films with a great uneveness. PA2 iii) Polycrystalline films predominantly having {100} planes and capable of being highly oriented can be obtained when formed with a large thickness and under specific synthesis conditions (keeping the concentration of a carbon source at a relatively high level). PA2 i) in the case when the nucleation density is low, monocrystals are formed, but are deposited at random in regard to orientation and in a granular form (the ratio of height to breadth is 1:3 or less, and usually 1:2 or less); PA2 ii) in the case when the nucleation density is high, polycrystalline films with a great uneveness are formed; and PA2 iii) although the {100}-oriented films have surfaces substantially parallel to the substrates and can have regions having a good flatness, they are essentially polycrystalline films and additionally a number of crystal imperfections (defects due to dislocation, twin planes, etc.) are present in the films.
The above conventional diamond crystals have been involved in the following problems.